In-plane switching LCD panel having different alignment layers

ABSTRACT

The present invention discloses an IPS LCD device including a first and second substrate opposing with each other, a common electrode on an inner surface of the first substrate, a pixel electrode parallel to the common electrode, a first alignment layer covering the common and pixel electrodes, wherein the first alignment layer is rubbed, a second alignment layer on an inner surface of the upper substrate, wherein the second alignment layer is photo-aligned, and a liquid crystal layer between the first and second substrates.

CROSS REFERENCE

[0001] This application claims the benefit of Korean Patent ApplicationNo. 2000-50124, filed on Aug. 28, 2000, under 35 U.S.C. § 119, theentirety of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the invention

[0003] The present invention relates to a liquid crystal display device,and more particularly to a liquid crystal display device implementingin-plane switching (IPS) where an electric field to be applied to aliquid crystal is generated in a plane parallel to a substrate.

[0004] 2. Description of Related Art

[0005] Recently, light and thin liquid crystal display (LCD) deviceswith low power consumption are used in office automation equipment,video devices, and the like. Such LCD's typically use an opticalanisotropy and spontaneous polarization of a liquid crystal (LC). Theliquid crystal has a thin and long liquid crystal molecule, which causesa directional alignment of the liquid crystal molecules. At this point,an alignment direction of the liquid crystal molecules is controlled byapplying an electric field to the liquid crystal molecules. When thealignment direction of the liquid crystal molecules is properlyadjusted, light is refracted along the alignment direction of the liquidcrystal molecules to display image data. Of particular interest is anactive matrix (AM) LCD, in which a plurality of thin film transistorsand pixel electrodes are arranged in the shape of an array matrix,because of its high resolution and superiority in displaying movingpictures. Driving methods for such LCD's typically include a twistednematic (TN) mode and a super twisted nematic (STN) mode. A TN liquidcrystal panel has high transmittance and aperture ratio. In addition,since the common electrode on the upper substrate serves as a ground,static electricity is prevented from destroying the liquid crystalpanel.

[0006] Although TN LCD's and STN LCD's, which have the same structure,have been put to practical use, they have a drawback in that they have avery narrow viewing angle. In order to avoid the problem of narrowviewing angle, IPS LCD devices have been proposed. IPS LCD devicestypically include a lower substrate where a pixel electrode and a commonelectrode are disposed, an upper substrate having no electrode, and aliquid crystal interposed between the upper and lower substrates. TheIPS LCD device has advantages in contrast ratio, gray inversion, andcolor shift that are related to the viewing angle.

[0007]FIG. 1A is a plan view illustrating in detail the structure of onepixel region in an IPS-LCD device, specifically, a unit pixel region 10.In addition, a cross-sectional view taken along a line “B-B” in FIG. 1Ais illustrated in FIG. 1B.

[0008] On the surface of a lower substrate 1 a adjacent to the liquidcrystal layer, a scan signal line 2 made of, for example, aluminum (Al)is formed extending along the x-direction, as shown in FIG. 1A. Inaddition, a reference signal line 4, also known as a common line, isformed extending along the x-direction, close to the scan signal line 2on the +y-direction side thereof. The reference signal line 4 is alsomade of, for example, Al. A region surrounded by the scan signal line 2,the reference signal line 4, and the video signal lines 3 constitutesthe unit pixel region 10.

[0009] In addition, the unit pixel region 10 includes a referenceelectrode 14 formed by the reference signal line 4 (identified as4(14)), and another reference electrode 14 formed adjacent to the scansignal line 2. The pair of horizontally extending reference electrodes14 are positioned adjacent to one of a pair of video signal lines 3 (onthe right side of the FIG. 1A), and are electrically connected to eachother through a conductive layer 14 a, which is formed simultaneouslywith the reference electrodes 14.

[0010] In the structure described above, the reference electrodes 14form a pair extending in the direction parallel to the scan signal line2. In other words, the reference electrodes form a strip extending in adirection perpendicular to the video signal lines 3, later described.

[0011] A first insulating layer 11 (see FIG. 1B) made of, for example,silicon nitride is formed on the surface of the lower substrate 1 a onwhich the scan signal lines 2 are formed, overlying the scan signal line2, the reference signal lines 4, and the reference electrodes 14. Thefirst insulating layer 11 functions as an inter-layer insulating filmfor insulating the scan signal line 2 and the reference signal line 4from the video signal lines 3, (b) as a gate-insulating layer for aregion in which a thin film transistor (TFT) is formed, and (c) as adielectric film for a region in which a capacitor “Cstg” is formed. TheTFT includes a drain electrode 3 a and a source electrode 15 a. Asemiconductor layer 12 for the TFT is formed near a crossing point ofthe scan signal line 2 and video signal line 3. A first polarizationlayer 18 is formed on the other surface of the lower substrate 1 a.

[0012] On the first insulating layer 11, a display electrode 15 isformed parallel with the reference electrode 14. One end portion of thedisplay electrode 15 is electrically connected to the conductive layer14 a, and the other end portion thereof is electrically connected to thesource electrode 15 a. Still on the first insulating layer 11, a firstplanar layer 16 is formed to cover the display electrode 15. A firstalignment layer 17 is formed on the first planar layer 16.

[0013]FIG. 1B also illustrates a cross-sectional view of the uppersubstrate 1 b on which a black matrix 300 is formed. A color filter 25is formed to close an opening in the black matrix 300. Then, a secondplanar layer 27 is formed to cover the color filter 25 and the blackmatrix 300. A second alignment layer 28 is formed on the surface of thesecond planar layer 27 facing the liquid crystal layer.

[0014] The color filter 25 is formed to define three unit pixel regionsadjacent to and extending along the video signal line 3 and to positiona red (R) filter, a green (G) filter, and a blue (B) filter, forexample, from the top of the three unit pixel regions. The three unitpixel regions constitute one pixel region for color display.

[0015] A second polarization layer 29 is also arranged on the surface ofthe upper substrate 1 b that is opposite to the surface of the uppersubstrate 1 b adjacent to the liquid crystal layer, on which variouslayers are formed as described above.

[0016] It will be understood that in FIG. 1B, a voltage applied betweenthe reference electrodes 14 and the display electrode 15 causes anelectric field “E” to be generated in the liquid crystal layer “LC” inparallel with the respective surfaces of the lower and upper substrates1 a and 1 b. This is why the illustrated structure is referred to as thein plane switching, as mentioned above.

[0017] With reference to FIGS. 2, 3A, and 3B, operation modes of atypical IPS LCD device are explained in detail.

[0018]FIG. 2 is a conceptual cross-sectional view illustrating a typicalIPS LCD device. As shown, lower and upper substrates 1 a and 1 b arespaced apart from each other, and a liquid crystal “LC” is interposedtherebetween. The lower and upper substrates 1 a and 1 b are called anarray substrate and a color filter substrate, respectively. On the lowersubstrate 1 a, pixel and common electrodes 15 and 14 are disposed. Thedisplay and reference electrodes 15 and 14 are positioned parallel withand spaced apart from each other. On a surface of the upper substrate 1b, a color filter 25 is disposed opposing the lower substrate 1 a. Thedisplay and reference electrodes 15 and 14 apply an electric field “E”to the liquid crystal “LC”. The liquid crystal “LC” has a negativedielectric anisotropy, and thus it is aligned parallel to the electricfield “E”. The display electrode 15 and reference electrode 14 are alsoreferred to as the pixel electrode 15 and common electrode 14.

[0019]FIGS. 3A and 3B illustrate operation modes for the typical IPS-LCDdevice shown in FIG. 2. For an off state, the long axes of the liquidcrystal molecules “LC” maintain some angle with respect to an invisibleline that is perpendicular to the pixel and common electrodes 15 and 14.The angle is 45 degrees, for example. At this point, the pixel andcommon electrodes 15 and 14 are parallel with each other.

[0020] For an on state, an in-plane electric field “E”, which isparallel to the surface of the lower substrate 1 a, is generated betweenthe pixel and common electrodes 15 and 14. The reason is that the pixelelectrode 15 and common electrode 14 are formed together on the lowersubstrate 1 a. Then, the liquid crystal molecules “LC” are twisted suchthat the long axes thereof coincide with the electric field direction.Thereby, the liquid crystal molecules “LC” are aligned such that thelong axes thereof are perpendicular to the pixel and common electrodes15 and 14. The liquid crystal used in the above-mentioned IPS LCD panelincludes a negative dielectric anisotropy.

[0021] Returning to FIG. 1B, the second alignment layer 28 is formed onthe second planar layer 27. The second planar layer 27 is an overcoatmade of acrylate-based or epoxy-based resin and serves to protect thecolor filter 25. In addition, the second planar layer 27 compensates forstepped portions due to the color filter 25 such that a platen surfaceis provided for the upper substrate 1 b having the color filter 25.After the second planar layer 27 and second alignment layer 28 aresequentially formed on the color filter 25, the second alignment layer28 is rubbed using a rubbing roller. Then, the second alignment layer 28has an alignment direction for “off state” of the liquid crystalmolecules “LC” shown in FIG. 4A. At this point, because the secondplanar layer 27 is relatively soft, it is stained during theabove-mentioned rubbing step such that an abnormal line pattern isformed thereon. If the alignment layer is stained, the liquid crystalmolecules are abnormally aligned in off state.

[0022] For the foregoing reason, there is a need for an IPS LCD devicethat prevents the above-mentioned stained error of the alignment layer.

SUMMARY OF THE INVENTION

[0023] To overcome the problems described above, the present inventionprovides an IPS LCD device, which has a structure that prevents thestained error of the alignment layer, and a fabricating method thereof

[0024] The present invention, in part, provides an IPS LCD device, whichincludes: first and second substrate opposing with each other; a commonelectrode on an inner surface of the first substrate; a pixel electrodeparallel to the common electrode; a first alignment layer covering thecommon and pixel electrodes, wherein the first alignment layer isrubbed; a second alignment layer on an inner surface of the uppersubstrate, wherein the second alignment layer is photo-aligned; and aliquid crystal layer between the first and second substrates.

[0025] The second alignment layer includes a photo-sensitive material.Specifically, the second alignment layer is a photolysis type polymer ora photo-crosslinkable polymer.

[0026] A dichroic subtraction of the second alignment layer is largerthan 0.025 inclusive.

[0027] A surface of the second alignment layer has a pencil hardness of1H to 5H both inclusive.

[0028] The first alignment layer has a greater anchoring energy than thesecond alignment layer has.

[0029] Liquid crystal molecules of the liquid crystal layer are at anangle of 0 exclusive to 3 inclusive with respect to the first and secondalignment layers.

[0030] The present invention, also in part, provides a method forfabricating an IPS LCD device. The method includes: preparing a firstsubstrate; forming a common electrode and a pixel electrode on the firstsubstrate, the common electrode and pixel electrode being parallel toeach other; forming a first alignment layer on the common and pixelelectrodes and rubbing the first alignment layer; preparing a secondsubstrate; forming a second alignment layer on the second substrate andphoto-aligning the second alignment layer; arranging first and secondsubstrates such that the first and second alignment layers oppose witheach other; and forming a liquid crystal layer between the first andsecond substrates.

[0031] A partially polarized ray is used for photo-aligning the secondalignment layer. The partially polarized ray has a polarization ratio ofabove 4 inclusive for 350 nm, or has a polarization ratio of above 8inclusive for 280 nm

[0032] Advantages of the present invention will become more apparentfrom the detailed description given hereinafter. However, it should beunderstood that the detailed description and specific examples, whileindicating preferred embodiments of the invention, are given by way ofillustration only, since various changes and modifications within thespirit and scope of the invention will become apparent to those skilledin the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus do not limit thepresent invention.

[0034] In the accompanying drawings, like reference numerals denote likeparts.

[0035]FIG. 1A is a plan view illustrating an IPS LCD device according tothe related art;

[0036]FIG. 1B is a cross-sectional view taken along a line “B-B” of FIG.1A;

[0037]FIG. 2 is a conceptual cross-sectional view illustrating a typicalIPS LCD device;

[0038]FIG. 3A is a perspective view illustrating “off state” of the IPSLCD device of FIG. 2;

[0039]FIG. 3B is a perspective view illustrating “on state” of the IPSLCD device of FIG. 2;

[0040]FIG. 4 is a cross-sectional view illustrating an IPS LCD deviceaccording to a preferred embodiment of the present invention;

[0041]FIG. 5 is an elevational view illustrating that a first alignmentlayer is formed on a lower substrate; and

[0042]FIG. 6 is an elevational view illustrating that a second alignmentlayer is formed on an upper substrate.

DETAILED DESCRIPTION OF PROFFERED EMBODIMENTS

[0043] Reference will now be made in detail to embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings.

[0044] In FIG. 4, a lower substrate 110 and an upper substrate 150oppose each other, and a liquid crystal layer 180 is interposedtherebetween. The lower and upper substrates 110 and 150 include a thinfilm transistor (not shown) and a color filter (not shown),respectively. On an inner surface of the lower substrate 110, a commonelectrode 120 and a pixel electrode 130 are formed with an intervaltherebetween. A first alignment layer 140 is formed over the lowersubstrate 110 such that it covers the common and pixel electrodes 120and 130. On an inner surface of the upper substrate 150, a planar layer160 and a second alignment layer 170 are sequentially formed. The planarlayer 160 is preferably made of acrylate-based or epoxy-based resin andserves to protect the color filter (not shown), which is formed on theinner surface of the upper substrate 150. The planar layer 160 furtherserves to platen the inner surface of the upper substrate 150 where thecolor filter (not shown) is formed, but may be omitted.

[0045] The liquid crystal molecules of the liquid crystal layer 180 arealigned with respect to the common and pixel electrodes 120 and 130. Thealignment state of the liquid crystal molecules depends on alignmentdirections of the first and second alignment layers 140 and 170. Thealignment of the liquid crystal molecules must be carefully controlled,because the alignment thereof affects a response property of the liquidcrystal molecules when an electric field is applied to the liquidcrystal molecules. The first and second alignment layers 140 and 170 arerubbed and photo-aligned, respectively, each to have an alignmentdirection such that they serve to control the above-mentioned alignmentof the liquid crystal molecules 180. To prevent a stained error of thesecond alignment layer 170, the second alignment layer 170 is formeddifferently from the first alignment layer 140, which is explainedbelow.

[0046] With reference to FIGS. 5 and 6, different methods of forming thefirst and second alignment layers 140 and 170 are explained,respectively.

[0047] As shown in FIG. 5, the first alignment layer 140 is formed onthe lower substrate 110 having the common and pixel electrodes 120 and130. To form the first alignment layer 140, a polymer, which ispreferably a polyimide-based material, is deposited and hardened on thelower substrate 110. Then, a surface of the first alignment layer 140 isrubbed using a rubbing roller 200 such that a plurality of microgroovesare formed in a uniform direction on the surface. The direction of themicrogrooves on the lower substrate surface is the same as the rubbingdirection of the rubbing roller 200. At this point, the rubbing roller200 has a much larger diameter than the height of steps due to thecommon and pixel electrodes 120 and 130. Therefore, the steps, resultingfrom the common and pixel electrodes 120 and 130 have little affect onthe above-mentioned rubbing process and the quality of the firstalignment layer 140 thereof.

[0048] At this point, it is very important to select a proper materialfor the alignment layer. If the first alignment layer 140 is too soft,the first alignment layer 140 is stained during the above-mentionedrubbing. Therefore, the first alignment layer 140 preferably has aproper hardness.

[0049] The hardness of the first alignment layer 140 can be examinedusing a pencil lead. That is to say, the first alignment layer 140 issequentially scratched using various pencil leads each having differenthardnesses. Then, the hardest pencil lead is selected among those thancan be scratched on the alignment layer. The pencil hardness thereof isassumed to be the preferred hardness of the alignment layer. At thispoint, the pencil hardness is measured according to Japanese IndustryStandard (JIS) K5400.

[0050] The hardness and density of a conventional pencil lead depends ona mixed proportion of graphite (black lead) and clay in the pencil lead.If the pencil lead is relatively hard, it is denoted as “nH”, wherein“H” is the first letter of “hard” and “n” is a number. Whereas, if thepencil lead is relatively soft, it is denoted as “nB”, wherein “B” isthe first letter of “black”. For a larger “n”, the hard pencil leaddenoted as “nH” is more harder and has a lighter color, whereas the softpencil lead denoted as “nB” is more softer and has a darker color. Inaddition, “HB” is used for denoting standard hardness and density, but“F” is used for denoting a medium hardness between “HB” and “H”. “F” isthe first letter of “firm”. The pencil lead has a hardness range of “9H”to “6B”, and is used for various purposes depending on its hardness anddensity.

[0051] Returning to FIG. 5, the first alignment layer 140 preferably hasa hardness of “1H” to “5H” both inclusive to be prevented from beingstained during the rubbing.

[0052] Next, as shown in FIG. 6, a planar layer 160 is formed on theupper substrate 150 having a color filter (not shown), and then apolymer is deposited and hardened to form the second alignment layer170. The second alignment layer 170 is a photosensitive orphoto-alignment polymer such as a polyimide-based polymer showingaligning property upon irradiation of light. At this point, a linearlypolarized ultra-violet ray is irradiated on the second alignment layer170 such that surface molecules of the second alignment layer 170 arealigned in a uniform pattern. A photolysis type polymer or aphoto-crosslinkable polymer is used as the photo-alignment polymer. Forthe photolysis type polymer, its high polymer main chain parallel to apolarization plane undergoes anisotropic photolysis by irradiation of alinearly polarized light. Whereas, for the photo-crosslinkable polymer,its high polymer side chains parallel to a polarization plane areselectively crosslinked to one another by irradiation of linearlypolarized light.

[0053] The above-mentioned linearly polarized ultra-violet ray may be apartially polarized one. In that case, a polarization ratio of thelinearly polarized ultra-violet ray is preferably at least above 4inclusive for 350 nm and above 8 inclusive for 280 nm. To calculate thepolarization ratio, a luminance of a polarized ray from a polarizer ismeasured using a photo-detector. That is to say, the photo-detector issequentially adjusted such that a polarizing axis of the photo-detectoris at first parallel and then perpendicular to that of the polarizer.Parallel luminance and perpendicular luminance are measured when thepolarizing axes of the polarizer and photo-detector are parallel andperpendicular to each other, respectively. Then, the polarization ratiois defined as the parallel luminance divided by the perpendicularluminance, which is shown in a first relationship.

[0054] Polarization Ratio=Parallel Luminance/PerpendicularLuminance—Relationship 1.

[0055] Though the second alignment layer 170 is photo-aligned using theabove-mentioned linearly polarized ray, it can show aligning propertiesfor liquid crystal molecules only if it has some anisotropy. Theanisotropy of the alignment layer is measured by polarizationexperiments using an ultra-violet/visible (UV/VIS) spectroscopy or aFourier transform infrared (FTIR) spectroscopy. A dichroic subtractionshows the level of the anisotropy of the alignment layer and iscalculated by subtracting a perpendicular absorbance from a parallelabsorbance. The parallel absorbance is measured when the polarized rayused for the photo-alignment has a parallel polarization direction thatis parallel to a light source of the spectroscopy. Whereas, theperpendicular absorbance is measured when the polarized ray used for thephoto-alignment has a perpendicular polarization direction that isperpendicular to the light source of the spectroscopy. Theabove-mentioned relationship is as follows.

[0056] Dichroic Subtraction=Parallel Absorbance−PerpendicularAbsorbance—Relationship 2.

[0057] At this point, the dichroic subtraction for the second alignmentlayer 170 of the preferred embodiment is preferably larger than 0.025inclusive.

[0058] After the first and second alignment layers 140 and 170 arerespectively formed on the lower and upper substrates 110 and 150, thelower and upper substrates 110 and 150 are arranged to oppose with eachother.

[0059] At this point, the alignment directions, or microgroovedirections of the first and second alignment layers 140 and 170 may beequal or may be opposite to each other. The microgrooves of the firstand second alignment layers 140 and 170 are inclined in some directionwith respect to the lower and upper substrates 110 and 150,respectively. Therefore, when liquid crystal molecules are interposedbetween the first and alignment layers 140 and 170, a first portion ofthe liquid crystal molecules facing an inner surface of the lowersubstrate 110 have a first pretilt angle corresponding to the firstalignment layer 140. Whereas, a second portion of the liquid crystalmolecules facing the upper substrate 150 have a second pretilt anglecorresponding to the second alignment layer 170. If the first and secondpretilt angles of the first and second alignment layers 140 and 170 arelarge, the alignment directions of the first and second alignment layers140 and 170 should be considered in arranging the lower and uppersubstrates 110 and 150. However, because most of the liquid crystalmolecules of the IPS LCD device rotate just on a plane parallel to thelower and upper substrates 110 and 150, the IPS LCD device adopts asmaller pretilt angle. Therefore, the alignment directions of the firstand second alignment layers 140 and 170 can be regardless duringadjusting the lower and upper substrates 110 and 150.

[0060] Specifically, a typical TN LCD device adopts about 4 degrees forthe pretilt angle, whereas the pretilt angle of the IPS LCD deviceaccording to the preferred embodiment is preferably larger than 0degrees exclusive and smaller than 3 degrees inclusive. If the pretiltangle of the IPS LCD device is very large, a light leak occurs in ablack state such that a contrast ratio is deteriorated. In addition, ifthe pretilt angle of the IPS LCD device is 0 degrees, operationqualities and electro-optic properties are deteriorated. Therefore, theabove-mentioned range of 0 degrees exclusive to 3 degrees inclusive ispreferred.

[0061] The first and second alignment layers 140 and 170, respectively,have first and second anchoring energies for aligning the liquid crystalmolecules with the above-mentioned pretilt angles. If the alignmentlayer has a larger anchoring energy, a larger restoring force acts onthe liquid crystal molecules after an electric field applied to theliquid crystal molecules is stopped. Preferably, the first alignmentlayer 140 has a larger anchoring energy than the second alignment layer170.

[0062] As explained above, the preferred embodiment of the presentinvention adopts the photo-aligned alignment layer 170 for the uppersubstrate 150 having the color filter such that the alignment error dueto the rubbing is excluded.

[0063] The invention being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. An IPS LCD device comprising: first and secondsubstrates opposing with each other; a common electrode on an innersurface of the first substrate; a pixel electrode parallel to the commonelectrode on the inner surface of the first substrate; a first alignmentlayer covering the common and pixel electrodes, wherein the firstalignment layer is rubbed; a second alignment layer on an inner surfaceof the upper substrate, wherein the second alignment layer isphoto-aligned; and a liquid crystal layer between the first and secondsubstrates.
 2. The device of claim 1, wherein the second alignment layerincludes a photo-sensitive material.
 3. The device of claim 2, whereinthe second alignment layer is a photolysis type polymer.
 4. The deviceof claim 2, wherein the second alignment layer is a photo-crosslinkablepolymer.
 5. The device of claim 1, wherein a dichroic subtraction of thesecond alignment layer is larger than 0.025 inclusive.
 6. The device ofclaim 1, wherein a surface of the second alignment layer has a pencilhardness of 1H to 5H both inclusive.
 7. The device of claim 1, whereinthe first alignment layer has an anchoring energy greater than thesecond alignment layer.
 8. The device of claim 1, wherein liquid crystalmolecules of the liquid crystal layer are at an angle of 0 degreesexclusive to 3 degrees inclusive with respect to the first and secondalignment layers.
 9. A method for fabricating an IPS LCD device, themethod comprising: preparing a first substrate; forming a commonelectrode and a pixel electrode on the first substrate, the commonelectrode and pixel electrode being parallel to each other; forming afirst alignment layer on the common and pixel electrodes; rubbing thefirst alignment layer; preparing a second substrate; forming a secondalignment layer on the second substrate; photo-aligning the secondalignment layer; arranging first and second substrates such that thefirst and second alignment layers oppose with each other; and forming aliquid crystal layer between the first and second substrates.
 10. Themethod of claim 9, wherein a partially polarized ray is used forphoto-aligning the second alignment layer.
 11. The method of claim 10,wherein the partially polarized ray has a polarization ratio of above 4inclusive for 350 nm.
 12. The method of claim 10, wherein the partiallypolarized ray has a polarization ratio of above 8 inclusive for 280 nm.